1. Technical Field of the Invention
The present invention relates to a radiation detector used in the fields of, for example, nuclear facilities, nuclear medicine diagnosis, atomic physics, and the like, and more particularly, to a beta ray detector and a beta ray reconstruction method.
2. Description of the Related Art
In the fields of, for example, nuclear facilities, nuclear medicine diagnosis, atomic physics, and the like, a radiation detector is used for detecting or measuring alpha rays (α rays), beta rays (β rays), and gamma rays (γ rays).
A radiation detector for measuring particularly gamma rays (γ rays) (the radiation detector will be hereinafter referred to as a “gamma ray detector”) is a laminate of a collimator, a NaI (sodium iodide) and a PMT (photomultiplier tube), in which a lead collimator with parallel micropores allows only gamma rays coming from the direction of the pores to reach single crystals of the NaI, whereby fluorescent light is emitted proportional to the amount of gamma ray energy deposited therein, and the PMT detects the light to detect the intensity and the position of the gamma rays.
The above-described gamma ray detector is used for measuring gamma rays (γ rays) emitted from radioactive medicines given to a patient in for nuclear medicine diagnosis purposes in a SPECT (single photon emission computed tomography) equipment and a PET (positron emission tomography) equipment.
As an example of the above-described gamma ray detector, proposals have already been made, for example, in Patent Document 1.
On the other hand, a radiation detector for measuring beta rays (β rays) (the radiation detector will be hereinafter referred to as a “beta ray detector”) is mainly used for detecting the contamination of workers, clothes, equipments, or the like in the nuclear facilities, and proposals have already been made, for example, in Patent Documents 2 and 3.
The gamma ray detector of Patent Document 1 aims to achieve a reduction in weight of a radiation detector and improve directivity and scanning properties, thereby providing accurate detection information.
For this purpose, as illustrated in FIG. 1, the gamma ray detector has a detection portion which is configured of a phoswich detector 51 composed of a plastic scintillator 51a and a CsI (Tl) scintillator 51b, in which gamma rays are incident to the scintillators to emit light pulses which are in turn converted to electric signals by a photoelectric converter 51c. The electric pulses are shaped and amplified by amplifiers 55 and 56 to detect the times of two arbitrary points at the rising parts of the pulses having different attenuation time by a constant fraction method. The time lag between the two points is input to a rise-time-to-height converter 57 converting the time lag to wave height and outputting the same and the light emission pulse of the plastic scintillator 51a is triggered to measure the light emission output of the CsI (Tl) scintillator 51b. That is to say, it is a basic principle that incident γ rays and the like are detected only when both of the two scintillators 51a and 51b emit light.
The beta ray detector of Patent Document 2 aims to provide a radiation detector capable of allowing a scintillator with a wide area to function effectively and having a low background count for gamma rays and uniform and high detection efficiency regardless of the incident positions of beta rays.
For this purpose, as illustrated in FIGS. 2A and 2B, the beta ray detector is equipped with a thin board-like scintillator 61 arranged on a measuring surface, a light guide plate 62 attached and optically connected to the backside of the scintillator 61 to guide light, a condensing portion 63 optically connected to each of the side faces of the scintillator 61, a photoelectric conversion element 64 optically connected to the condensing portion 63, and a signal processing circuit 65 connected to the photoelectric conversion element 64.
The beta ray detector of Patent Document 3 aims to provide a radiation detector capable of detecting a radioactive substance existing in a measuring portion based on a signal associated with beta rays when the radioactive substance emits beta rays and other radiation.
For this purpose, as illustrated in FIG. 3, the beta ray detector has a detection portion 70 which is provided with a first scintillator 71 that emits light in response to an incidence of beta rays and other radiation (for example, gamma rays) and a second scintillator 72 that is shielded by a beta ray impermeable substance 79 and emits light in response to an incidence of radiation other than beta rays. Moreover, a calculation portion detects a radioactive substance existing in the measuring portion by a calculation using a signal associated with the first scintillator and a signal associated with the second scintillator.
[Patent Document 1]    Japanese Patent Application Laid-Open No. 5-066275; “Directional Variable Radiation Detector”
[Patent Document 2]    Japanese Patent Application Laid-Open No. 2002-341036; “Radiation Detector”
[Patent Document 3]    Japanese Patent Application Laid-Open No. 2007-010332; “Radiation Detector”
Gamma rays (γ rays) are electromagnetic waves having a wavelength shorter than about 10 pm, produced due to a displacement of an energy level within an atomic nucleus and are characterized in that they exhibit extremely high permeability. For this reason, a thick board or a laminated member of an inorganic material (for example, CsI, NaI) through which gamma rays can hardly permeate is generally used as a scintillator of a gamma ray detector.
On the contrary, beta rays (β rays) are electrons or positrons which are emitted when an atomic nucleus (neutron) undergoes beta decay and are characterized in that they exhibit extremely low permeability. For this reason, an inorganic material having a high rate of later-described “backscattering” is not used as a scintillator of a beta ray detector, but an organic material (for example, a plastic scintillator) having a high rate of absorption with respect to beta rays is generally used.
The above-described gamma ray detector of Patent Document 1 is the phoswich detector 51 composed of the plastic scintillator 51a and the CsI (Tl) scintillator 51b, and therefore, only the light emission output of the CsI (Tl) scintillator 51b is used for gamma ray detection.
Moreover, the beta ray detector of Patent Document 2 detects only the light emission output of the thin board-like scintillator 61 via the light guiding plate 62.
Furthermore, the beta ray detector of Patent Document 3 detects the radioactive substance existing in the measuring portion from the light emission outputs from both the first scintillator 71 and the second scintillator 72 in a state where areas between the first scintillator 71 and the second scintillator 72 are shielded by the beta ray impermeable substance 79.
For example, when an organic material (for example, a 4 mm-thick plastic) was used as a scintillator for detecting beta rays in an energy range of 0 to 3 MeV, similar to the conventional case, a detection efficiency of 90% or higher was obtained for an energy range of 0 to 1 MeV; however, there is a problem in that the detection efficiency decreases abruptly in an energy range exceeding 1 MeV and becomes 10% or lower in an energy range of 2 MeV, for example.
Moreover, an organic scintillator has a small amount of fluorescence per energy and hence has a low energy resolution compared to an inorganic scintillator.
On the other hand, when an inorganic material (for example, 4 mm-thick NaI) was used as a scintillator, there is a problem in that it is only possible to obtain a detection efficiency of less than 80% over the entire energy ranges of 0 to 3 MeV.